Yeah, you really need to pull a deep vacuum. You have to lower the pressure inside the system to the point where any water boils off at ambient temperature.

There are several reasons for this step. First, air and water are not refrigerants. Air will not condense, so does not assist in cooling. It does stress the compressor, consume more energy, etc., but for no value. Second, the water reacts with refrigerants to form acids which over time eat away at the system. This is called an acid eat out. Third, excess water can freeze inside the system and block the flow of refrigerant. This results in cyclic operation of the system - very cold, then very warm, then very cold when the ice melts....

I don't think you can have it vacuumed and then drive home to fill it - the vacuum will be lost when the hoses are disconnected from the system. You may be able to rent a vacuum pump and guages from a rent-all type of place. I've since acquired my own equipment, but that's what I used to do.